The Direction of Value Flow in Open Multi-service ... - Semantic Scholar

The Direction of Value Flow in Open Multi-service Connectionless Networks Bob Briscoe BT Research, B54/130, Adastral Park, Martlesham Heath, Ipswich, IP5 3RE, England Tel. +44 1473 645196

20 Aug 2000

Abstract

brought the issue of reapportionment of charges between the end customers back into the limelight [7]. However, to tackle this issue, the traditional model This paper argues that all network providers in of a unit of communication value is inadequate bea connectionless multi-service network should ofcause it grew in the context of connection-oriented fer each class of their service to each neighbour for telephone systems. Worse, it has often been misapeach direction at a single price. This is called ‘splitplied to the connectionless Internet. edge pricing’. If sets of customers wish to reapportion their networking charges between themselves, The connection-oriented mind-set also leads to conthis should be tackled end-to-end. Edge reappor- fusion over blame and liability for each unit of comtionment should not be muddled with networking munication. This paper explicitly clarifies these charges, as is the case in the telephony market. fundamental issues. Our minimalist model for Avoiding the telephony approach is shown to offer a connectionless network business has boundaries full reapportionment flexibility, but avoids the oth- that match the service access points above and beerwise inevitable network complexity, particularly low the network layer of the OSI stack [23]. This for multicast. ‘Split-edge pricing’ is recursive, ap- business can be modelled as buying in lower and plying as much to relationships between providers higher layer services (e.g. links, virtual connections as to edge-customers. Various scenarios are dis- or naming services). It still applies to ISPs that cussed, showing the advantage of the approach. use their own links and services — the cost just These include phone to Internet gateways and even becomes internalised. inter-domain multicast conferences with heterogeneous QoS. The business model analysis suggests a This paper proposes a simple charging model that new, purely financial role of end-to-end intermedi- can be applied between any pair of multi-service connectionless networks for each class of service and ary in the Internet industry. for send and receive separately. It works whether the pair are both providers or even if one is an edge Keywords: Charging, pricing, end-to-end, clear- customer. The model’s simplicity ensures charging ing, Internet, business models. will always be straightforward at every border in the Internet, whether for unicast or multicast flows. No matter how many networks are connected together, any one network is only dependent on prices 1 Introduction from its direct neighbours. Therefore, the model is intrinsically scalable. We call the model ’split Traditionally, data communications has been sold edge pricing’. From the end customers’ points of so cheaply that charging for it on usage basis has view, this means that any flow through the Internot seemed feasible or sensible. While flat-rate net is sold on entry and on exit. As a consequence, subscription or connect-time charging prevails, the the model appears to restrict all end customers to question of reapportioning the value of a partic- each have to pay the price of their local network ular communication between its ends rarely sur- provider. This appears to restrict any customers faces. With the possibility of variable quality of who would rather reapportion the costs differently service (QoS) approaching, the need for some form between themselves (termed clearing). of usage-charging for high QoS has arisen. This has led to new thinking on cheaper usage-charging sys- Invariably, network providers offer their services at tems for packet networks [18, 4], which in turn has a set price regardless of the value each customer c British Telecommunications plc, 2000

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The Direction of Value Flow

derives from each transmission. This is a natural consequence of a competitive market, often called a ‘buyer’s market’. If usage-based charging is in operation, no one bothers with any communication of less value than this market price. However, transmissions naturally have at least two ends. (In fact, we consider two-ended flows as just a specific case of multipoint flows.) Often a transmission just never happens because one of the ends derives less value than their local price. Often in such cases, the total value derived from the transmission by all ends would have been greater than the total charges levied by all providers on all end customers. Therefore, it is in any network provider’s interest to matchmake customers who derive surplus value with those who would otherwise be in deficit. That is, clearing plays an important role in encouraging network demand. Matchmaking in the traditional telephony market is well understood. Various ways are available for end customers to share the cost of a call besides the normal ‘originator pays’. Examples are ‘calls free to the originator’, ‘local charge only’ etc. Telephony interconnect arrangements ensure that wherever payment enters the system, it ends up being cleared between the providers who bore the cost of each call. However, the interconnect pricing scheme that drives clearing blurs the distinction between clearing of edge payments and the market price of interconnect. This paper argues that these overcomplicated clearing arrangements are the result of evolution from a fully connected matrix of single country providers and are flawed for the Internet. Instead we propose ’split-edge pricing’ as a more flexible replacement. The apparent problem of no flexibility to clear between the ends is solved simply. Clearing can be achieved end-to-end, directly between customers or their edge providers, bypassing the core network businesses. If instead, clearing follows the same path as the data flow, we show that core network complexity becomes inevitable, particularly for multicast, but also for unicast. Incidentally, end-to-end clearing was never possible on the PSTN because there was no convenient way to form independently routed end-to-end data connections simultaneously to call progress. Clearly, this is possible on the Internet.

third party across the ends of a communication, leaving the ‘bill’ as just the balance of those charges that are directly retailed between the edge-provider and its edge-customer. E-commerce-based clearing allows part of local customer A’s usage to be ‘wholesaled’ to remote customer B or to third party C. While part of customer B’s usage can be wholesaled to customer A and so on. The cost of the act of clearing is significant; therefore it is important that the default apportionment in the core model matches the most common case. We establish that the common case is where both senders and receivers pay, at all ends of each transmission. This incidentally causes the perceived need for the network to report how many receivers are subscribed to a multicast to evaporate. We then consider the unpleasant fact that, on the Internet, a receiver can never protect itself from being sent to. We suggest a rather novel business model that is still optimised for the common case, but simultaneously has no receiver liability. For completeness, we examine the specific problems with the traditional clearing model used in telephony. The paper draws to a close by working through some example scenarios to suggest how the models would work in practice. Finally, limitations and further work are listed before conclusions are drawn. This paper is the union of the content of [3] and [2].

2

Related Work

Some authors state that they believe the business model of the current fixed access rate Internet is ‘sender takes all’ [12, 24]. This phrase is used to imply the sender’s ISP receives all the revenue. This is completely erroneous. ISP’s rates relate to access bandwidth, regardless of the direction in which it is used. Thus, ‘sender and receiver each take half’ is more appropriate (approximately). This is a similar position to the half-circuit charging common for data links, but applied end-to-end. MacKie-Mason et al asserts that the blame for a transmission is impossible to determine at the network level [16], an argument that can descend into sophistry. However, later, using precise definitions of the terms, we argue that the sender is always to blame for a Clearing requirements will differ on a per session transmission in a connectionless network. basis, therefore the model where clearing takes place end-to-end involves per-session accounting Clark analyses the apportionment of charges bewithout involving the network providers along the tween senders and receivers [7], and proposes an data path (other than those at the edge). Thus, engineering solution, which he admits would init seems natural for clearing to re-use existing e- troduce considerable complexity to the Internet if commerce concepts and mechanisms. This results implemented. Shenker et al describes edge pricin a scenario where the traditional telephone bill ing [19], a business model that appears regularly in becomes an anachronism for the Internet. Instead, communication networks and which forms much of edge-provider charges can be settled by any other the background to this work. 2 of 20

c British Telecommunications plc, 2000


3

The value of place data flow

The value of communication concerns the increservice mental value of having information in a certain place (or places) by a certain time, instead of or price ISPs as well as the original places. Usually, the more the information is worth, the more value is placed B C on having it in the right places in a timely manner. There is no value to the customer at all while customer A information is in transit. It is delivery that is imend-to-end pricing role portant. Strictly one also has to take account of the mitigating cost of storage in both places (or only in the second place if the sender deletes after sending). In summary the added value of transmission is the Figure 1: ‘End-to-end pricing’ role marginal change in value caused by associating a new location at the delivery time with the intrinsic value of the information to the customer. are not saying ISPs shouldn’t offer end-to-end pricing — it is clearly in their interest to matchmake beHowever, because the data communications market tween customers with surplus value and those with is fairly competitive, charges for communicating indeficit. All we are saying is that, if they do, end-toformation tend instead to follow the ‘cost plus marend pricing should be considered as a separate role gin’ rule. This is particularly so because it is very (Fig 1). Such a role could be a separate business difficult for providers to predict what value their — it could gain on some combinations and lose on customers put on moving any one piece of informaothers, possibly making a profit overall. In this case tion. it would be a retail service that used the networkAny payment to an edge-network provider has the ing services as wholesalers. It is also possible that two aspects — ‘who pays’ and ‘who is paid’. ‘Who edge customers could effectively take on this role is paid’ can only be each local provider collecting themselves. Fig 1 shows three end customers using its local price. With competitive ‘cost plus’ pric- a data path through multiple connected ISPs. The ing there is no scope for any provider to break relative value of the service flows and prices for one out of that. But, because communications nat- direction of one class of service is represented by urally involves at least two parties, in order to the thickness of the arrows. Note that the size of cover the total costs of all the providers involved, the proportions of prices represents a choice by the ‘who pays’ can be on a different apportionment. end-system that is willing to pay more than its local The edge customers do know the value to them price. In fact, the end-to-end pricing role may adof having the information at a certain place in vertise identical prices to each customer. However, time. Thus, although apportionment is difficult these could be modified by an offer by A to cover for network providers, it is very relevant to edge- a proportion (possibly all of it). Pricing between customers. Clearly, the network providers can stim- providers is omitted for clarity (but see later). ulate more use of their networks by making arrangements for customers to efficiently apportion costs Telephony firms have traditionally offered end-toend pricing because they are selling an application. between themselves. The role of network provider has always been muddled with selling the end-to-end application. This is already putting considerable strains on the In4 End-to-end pricing ternational Accounting Rate System (IARS) [14] with potentially s(n − 1)2 prices having to be neIf a price is higher than the perceived value for gotiated (where n is the number of edge providers any customer, she is free to get the remote party and s is the number of global schemes for sharing (or anyone else) to make up the difference through the proportions of the price between the ends, e.g. some higher level arrangement. On the other hand, local rate only, free to sender). In practice, end if the value to her is higher than her local price, she providers are grouped together to reduce the numis also free to offer to cover some of the costs of the ber of prices presented to customers. The PSTN remote end(s). However, our minimalist provider uses addressing conventions (e.g. +800 for free to doesn’t have to be concerned with matchmaking sender), but this limits commercial flexibility to the multiple customers to get round local discrepancies few schemes that are widely recognised. Clark probetween price and customer value. This is an issue posed an Internet-based solution to allow flexibilthat can be dealt with end-to-end, not locally. We ity [7]. However, catering for various combinations c British Telecommunications plc, 2000

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of sender and receiver payments through the core of the network needs packet format changes and router involvement. Further, wholesale prices between providers would have to be negotiated for every possible scheme for sharing charges between ends as well as for every possible grouping of end points beyond that boundary. Worse still, interprovider accounting would then require traffic flows to be isolated then further sub-classified by how much each end was paying on a per-flow basis. 2

The ‘n problem’ would still exist for our end-toend pricing solution but this is fairly easy to contain by grouping. An example scenario is given at the end of the paper. Importantly though, end-to-end pricing gets rid of all the inter-provider problems described above. There is no longer a need to identify end-to-end flows at inter-provider boundaries. Thus inter-provider charging could be based on bulk measures like average queue lengths, number of routing advertisements etc. Also, most importantly, end-to-end pricing can be introduced without changing the Internet at all, and it allows future flexibility. To summarise so far, we should ensure any discrepancy in the willingness to pay across end customers is normalised end-to-end first, so that edge ISPs always receive payment at their local price.

5

Common case value apportionment

Although we have delegated the problem of apportioning sender and receiver payments to a higher layer, it is still important to cater for the common case at the network charging level so that the higher layer functions are unnecessary in most cases. We propose that all edge providers should charge their local customers for both sending and receiving as the default case. Our primary justification for this stance is that the large majority of communication occurs between consenting parties. In this section we also argue that other possible default scenarios (e.g. ‘only senders pay’) would be unstable anyway and collapse back to our proposed model, which appears stable. Further, delegating charge reapportionment to a higher layer eliminates all but local pricing, so that we can extend charging for sending and receiving recursively to apply at the boundary between any pair of providers. This greatly simplifies inter-provider metering — essential for Internet scalability as QoS and multicast are introduced. Thus our edge pricing model applies to any ‘edge’ — whether at the edge of the Internet or just the edge of a backbone. The stability analysis intrinsically applies equally to this more general 4 of 20

Nd

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Figure 2: Split-edge pricing

case. Incidentally, allowing for prices for each direction to be different hardly needs justification. It allows for asymmetric costs (e.g. access technology like xDSL or satellite) and for asymmetric demand (e.g. some ISPs might host more big senders, while others might host the mass of receivers). If these factors aren’t asymmetric, the two prices can simply be set to be the same. Fig 2 shows a generic scenario with multiple networks, N, all connected to the network of interest, Nb . Each connected network has a status relative to Nb based on whether it provides more or less connectivity to other hosts at that class of service. Although the diagram gives the impression that Nb is a backbone network, any one of the neighbouring networks could be a simple link to an edge customer’s single host. The model is designed to be general enough for Nb to be an edge customer, an edge network, a backbone network or some hybrid. Those networks with the same suffix are of similar status relative to Nb . For instance, those labelled Nc may be edge customers, Nd may be equally large backbones and Ne a peer network. In fact, this is a simplification. To be more specific we propose that a provider should offer each class of service in each direction at a separate price. Thus, Fig 2 shows the situation for one of possibly many classes of service. Class of service is defined as a unique combination of the service mode (unicast, multicast) and quality (latency, instantaneous bandwidth, reliability, jitter). Quality specifications within one class may leave one parameter to be specified by the customer while others remain fixed, thus generalising both RSVP and diffserv [22, 1]. Appendix A justifies treating each class of service independently. Appendix B gives the full model for each class of service, but allows heterogeneous QoS per leg of the multicast. However, all this detail would obscure the summary of the analysis we attempt to give here, which we now continue. c British Telecommunications plc, 2000


A packet of a particular class of service is shown being multicast from Na into Nb and onward into the other networks. Because multicast is a general case of unicast this allows us to model both topologies. We will also be able to treat the topology as aggregation1 by reversing the direction of transmission. The term packet is used, but the arrows could represent flows of similar class packets for a certain time. Fig 2 highlights the pricing between networks Na and Nb . Wbas and Wbar denote the per direction weightings applied to the ’nominal charge’ that Nb applies to Na (for more detail on exactly what the nominal charge means, see Appendix B). Wabs and Wabr likewise weight the charge Na applies to Nb . Each weighted price is for transmission between the edge in question and the remote edge of the Internet, not just the remote edge of that provider. For full generality, there have to be four price weightings like this for every class of service at every inter-network interface, but the weights would take different values unless the neighbours were of the same status. The relationship between any two parties across the edge of their networks is split into prices for each class of service and each of these is further split into two prices for each direction, each of which are again split into ‘half’ prices that each party offers the other. Hence, we call this model ‘split-edge pricing’.

to different providers. Income appears to remain stable, but the source of the income switches from retail customers to interconnect causing the interprovider link to become a bottleneck. Thus costs increase without any increase in revenue. ’Only senders pay’ is also unstable where multicast is concerned. (’Only receivers pay’ is all but meaningless for multicast.) To support an ‘only multicast senders pay’ policy, all domains have to trust each other to faithfully report receiver community size. In Appendix B we show that it is simple for a domain to lie about its local receiver community size to increase its profits. Proposed mechanisms such as ‘EXPRESS count management protocol’ [11] suffer from this flaw. Solving this problem is unlikely to be successful without breaking the scalability benefits of receiver initiated IP multicast that ensure upstream nodes are unaware of downstream join and leave activity.

In contrast, ‘both senders and receivers pay’ is stable in both unicast and multicast cases. It also doesn’t lead to inefficient network utilisation unlike the above cases. It is also possible to cater for different balances of predominant senders and receivers by weighting the sending price differently to the receiving price. For instance if there are a few big predominant senders but many small predomThus the payment for traffic in any one direction inant receivers, the economy of scale in managing across each interface depends on the difference be- a large customer can be reflected in a lower sender tween the two weighted prices offered by the net- weighting. Similarly, the inefficiencies of multicasts works either side. In other words, no assumptions to small receiver communities compared to multiple are made about who is provider and who is cus- unicasts can be discouraged by slightly weighting tomer; this purely depends on the sign of the differ- multicast sender pricing. The aggregation case is ence between the charges at any one time. Clearly, similar, with ‘both senders and receivers pay’ staedge customers (Nc , say) have no provider status ble while the two other policies go unstable for the in the networking market. So, for all j, Wcjs = 0 same reasons as for multicast, but swapped round. and Wcjr = 0. If end customers want a different apportionment In Appendix B we analyse policies like ‘only senders of charges, we have made the case for this being pay’ or ‘only receivers pay’ using the model (by arranged end-to-end. The remainder of the paper simply setting all receiving weights to zero or all concentrates on issues surrounding clearing. Also, sending weights to zero). Stability of a policy is de- at the end, various worked examples are given to iltermined by assessing whether one network would lustrate how it would be achieved in practice. Howgain from a maverick policy. The results are sum- ever, first, we introduce one further relevant issue — that of how a receiver can control its costs, if it marised here. can’t stop itself being sent to. ’Only senders pay’ or ‘only receivers pay’ are only stable policies if all providers agree to adopt the same policy, and none break ranks. As soon as 6 Blame, liability and control one goes maverick, customers who are primarily receivers and those who are primarily senders migrate We have shown that all ends paying is the com1 Examples of packets that are forwarded until aggregamon case and a stable one so should be the default. tion (reverse multicast or ‘concast’ — converging cast) are: We can share the cost differently at higher level if • RSVP receiver initiated reservation (RESV) messages; end user value is shared differently from this de• pragmatic general multicast (PGM) [20] negative acfault (and it is worth bothering given the cost of knowledge (NACK) messages or the ‘lay breadcrumb’ another financial transfer). However, we must remessages[8] suggested in their place. member that a sender can decide not to send but c British Telecommunications plc, 2000

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a receiver can not avoid being sent to (in the current Internet). We must be careful here to define the context of the question of blame. We are only concerned with blame for sending into or receiving from the service access point of the network layer. Clearly, if someone operates a Web service, they don’t normally decide whether to send replies on a request by request basis. But this doesn’t mean they have been forced to send at the network level . They have chosen to put the service on a wellknown port with public access. They can stop certain people requesting them to send by securing the Web server or interposing a firewall. But, whenever they send it is because they have arranged it to be so.

7

End-to-end clearing

We have discussed how prices can be apportioned between the ends of a communication. We now discuss how payment will follow the same path. We can assume electronic commerce will make it possible for anyone to pay anyone else’s ISP on the Internet, even if a clearinghouse is needed We shall call this the ‘end-to-end clearing’ model (Fig 3). These arrangements will typically be made through higher level protocols. The act of making a financial transfer has similar order costs to the cost of transmission of a couple of small e-mails. In addition there is a the cost to the ISP of provision of processing resources for authentication. ThereUltimate sender blame presents a problem. In cases fore, arranging a different apportionment of charges where the sender derives surplus value from a com- between ends is more likely for long-lived sessions, munication and the receiver derives less value than such as Internet telephony or conferences on the their provider charges, receivers are vulnerable to mbone, than short connections, such as are typical being exploited (e.g. adverts). Such cases are much on the Web. However, a collection of related short rarer than it first appears, mainly because of con- connections may be combined into one longer-lived fusions that can be cleared by considering the fol- session for these purposes. lowing factors: In the ‘end-to-end clearing’ model, the clearing• The value of the information isn’t relevant house role deals with the end-to-end ’half-circuit’ when considering the networking service — sharing (including the straightforward price differonly the value of moving the information — ences between the two ends) leaving inter-provider accounting to be purely about wholesaling. The getting it to a useful place figure shows one end paying and follows example • Often the value of moving information is tran- proportions of this money as they are distributed sitory — getting it to a useful place to discover among the providers. The clearing role may be inthat moving it wasn’t useful volved in taking payments for higher level service from each customer (e.g. conference fees or pay• Often the value of moving lots of information TV charges). In such cases it knows the number is to get a small part of it to a useful place, of participants and can charge the customer on the but it isn’t possible to know which part before left (who is paying for everyone) accordingly. In moving it this example each leg is charged at fifty units and • The cost of transmitting information is often no profit is made. Note that inter-provider money far less than the cost of the effort of targeting flows match the flow of networking service in the opposite direction across the same interface. This which information should be transmitted is the deliberate aim of the model, to ensure that • Information in one direction often controls the bulk measurements at these boundaries can drive flow of information in the other interconnect charges based solely on local conditions. It also allows each pair of providers to choose Nonetheless, genuine cases remain where the retheir own basis for metering independently of other ceiver is being persistently forced to pay for transarrangements at other interfaces. mission that is valuable to the sender but not to the receiver. The only solution to this seemingly There is nothing to stop providers or customers intractable dilemma is for it to be customary for assuming the clearinghouse role, but the accounting all ends to pay, but the ultimate liability should information model needs to be based on a third remain with the sender. Any receiver could then party clearing system to allow for the most general dispute the customary apportionment (end-to-end) case. To clarify, the paying customer may make with no risk of denial (unless the sender had proof payment: of a receiver request). A similar but opposite situation used to prevail with the UK postal service. It • either to a dedicated clearing house was customary for the sender to pay for the stamp, but if it was missing or insufficient the receiver was • or direct to the ISP at the remote end (the liable for the payment, because the Royal Mail had remote customer need only notify her ISP’s an obligation to deliver every letter. payment interface to the payer) 6 of 20



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Figure 3: ‘End-to-end clearing’ model

Figure 4: ‘Iterative clearing’ model

• or even direct to the remote customer so that (or per packet) basis. For some flows, there may she can pay their own ISP even be proportional sharing of costs between the ends. For business model flexibility an accounting In all cases, the role of clearing must be separate system would need a ‘payee percentage’ field — even if there is no separate enterprise to achieve the the percentage of the total cost to be paid by the function. Note that the last case is special — the customer at the end being accounted for. Usually clearing role is null, but it still appears in the init would be 100% or 0% in the typical cases of formation model. In other words, the charges for ‘paid completely to local provider’ or ’completely all ends should never be lumped together while ac- to remote’. The balance would be the remote end’s counting. If, instead, end-to-end half-circuit shar- payment. Note, though, that the perceived purpose ing were achieved through the provider chain, end- of this model is the transaction efficiency when the to-end clearing information would have to be iden- local payee gets 100%. tified separately from that needed for wholesale accounting. If clearing information were not identified If Fig 4 is compared with the end-to-end clearing separately, the types of model that could be built model in Fig 3, both models end up with two of the edge ISPs paid the same amounts on a halfon the infrastructure would be restricted. circuit basis (we will explain where the third leg of the multicast has gone later). The difference is merely in the route the payment takes from payer to 8 Iterative clearing payee. With ‘iterative’ clearing the payment follows the data path. Along the way, providers take their We have presented what we believe to be an op- cut with two types of money sharing being mixed timum business model, but other models need to together: be considered. In particular, we will now consider a model similar to the public ’phone service, which • wholesale cut has one or two implicit features that need to be separated out for full understanding. We will consider • half-circuit sharing payment in the model first, rather than pricing, as it will then be easier to understand the pricing isHowever, the amount deducted from the flow at sues. each boundary doesn’t match the level of service In this model, ISPs don’t expect payment for all crossing that boundary. This can lead to complexsent and received traffic to be made to all edge ity in the network, as there is pressure to design providers (Fig 4). Instead a customer might pay the network itself to reveal the apportionment of their own provider on behalf of both (all) ends as costs. This was why Clark was concerned about in the normal case for telephony and as proposed by how much complexity would be added to the InClark for the Internet [7]. This alternative business ternet to cater for arbitrary combinations of sender model allows customers to decide into which end(s) and receiver payments. This is also why internapayment enters the system, on a per flow basis. We tional and interconnect on the PSTN have limited shall call this model the ‘iterative’ model for reasons flexibility to arbitrarily apportion charges between that will become clear as we go. The financial flows the ends. Even ‘free to sender’ calls are blocked bebetween providers in this model depend on at which tween a lot of countries because they don’t yet have ends payment is entering the system on a per flow prices set or the interconnect accounting in place. c British Telecommunications plc, 2000

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• If multicast is to be catered for by iterative clearing (e.g. conferences), each provider needs to know how many ends they are serving locally, both to inform the person paying and check settlement. In contrast, with the endto-end clearing model, if senders and receivers all pay, no-one needs to count ends nor trust others to count ends for them. If clearing is desired by the ends, only the ends need to know how many ends there are to pay for — no-one needs to calculate how many ends are attached to each provider. Thus, for instance, if there is a charge to join a conference, this can cover the cost of paying each participant’s communications charges as well as the content (each participant would have to declare their ISP when they join). The more who pay the host to join, the more there is to cover charges. Then the host can send bulk payments to the relevant ISPs, either directly or through a single clearer (see worked example below). Multicast has been omitted from Fig 4 simply because there is no generic way to handle multipoint networking with iterative clearing.

Specifically, there are five points stacked up against the ‘iterative clearing’ model: • As already pointed out, a ‘payee percentage’ field would have to drive inter-provider accounting, whether it was in accounting messages or packets. Otherwise the revenue of an edge ISP and its upstream providers would depend on a factor completely outside their control — to which end its customers chose to make payment. The ‘payee percentage’ field would therefore have to be trusted by upstream providers. To help prevent the field being tampered with, it would need to be signed by the remote ISP. How signed fields can be aggregated without losing the signature integrity would be a matter for further research. • Still further complication might be introduced for some future applications if the share of payment between the parties wasn’t fixed but depended on characteristics of the flow or other parameters only understood at a higher level — higher than the provider would normally be interested in. • Worse still, the payment should ideally be split taking into account the current prices of all the edge providers who will eventually be paid. The only alternative (used in the international accounting rate system (IARS) for telephony) is for ISPs to agree compromise prices between themselves that average out price inconsistencies. This is what has been causing all the tensions in IARS as some countries liberalise earlier than others causing huge variation in prices around the world, between which no happy compromise can be found. This is difficult even for a system where every end to end path only passes through two international carriers at maximum, each pair setting compromise prices with each other. With eight ISPs on many end to end Internet paths, five typical [17] and considerable peer interconnection, it is likely that it will take longer to negotiate prices than the time available, thus leading to distortions to providers’ supply and demand signals.

The only advantage of the ‘iterative’ model is that it appears to reduce (by one) the number of transactions to achieve the desired apportionment. Also all the inter-provider transactions can be fairly lightweight because they can be batched up. For example, consider the case where both the parties in an Internet ’phone conversation are being paid for by the caller. It appears less complex for the caller to pay everyone’s payments to her own ISP, then let the ISP transfer the correct amount to its upstream provider as part of a bulk transaction. However, on the other side of the bargain is a considerably more complicated network, compromise pricing, increased credit time lags and less flexibility in inventing new ways to apportion charges, particularly for multicast.

• Finally, because of the much longer provider chains typically found on the Internet, unacceptable delays will be introduced before the revenue arrives in the correct place. Any delay in clearing hugely increases the cost of the payment system, as extra trust mechanisms have to be invoked while the payment remains unconfirmed. These trust mechanisms have to be applied to the edge customers, not just the providers, therefore hugely increasing the total cost of the system.

Let us assume some way has been invented for an ISP’s edge customer, Ca , to announce her intention to cover some part of the transmission costs of parties communicating with her, Cb , Cc etc. Some suggestions are given in [7]. Kausar suggests modifications to SDP [9] to achieve this for longer sessions [15]. A price needs to be set and settlement made between Ca and each party. If this is achieved, end-to-end, between the parties involved there are no further engineering implications — the pairs of parties clearly trust each other

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9 9.1

Example scenarios Finding an end-to-end price



enough to enter into a financial arrangement and are willing to accept the cost of the transaction. However, there will be many occasions where the parties have no trust relationship. In these cases the problem reduces to, Cb , Cc etc finding suitable intermediaries. First they must know Ca ’s ISP. Ca may have already given this information in the session description protocol. Alternatively a directory of ISPs could be operated by the Internet address allocation registry (IANA) or any private concern, in which one could look up the network address of Ca and be given the payment interface of the associated ISP. This directory might be operated by one organisation monolithically (feasible for the current 74,000 ISPs) or it could be hierarchical like DNS. They may then choose to check whether their own ISP has a direct relationship with Ca ’s ISP. Alternatively, they could go straight to a different directory we postulate would be necessary. This directory would accept lists of ISPs and return a list of organisations that would act as an intermediary between them all. The mechanism would be identical to a Web search engine — an invert index of ISP-intermediary pairs that would accept queries of logically ANDed ISPs. There may be some intermediaries who will deal with any combination of ISPs through their own network of secondary clearing arrangements.

and received traffic, whoever is expected to pay for that usage. That is, we require the edge ISP to be willing to sign the relevant accounts if asked. Whoever ends up paying, the price applied to each account will be that set by the edge-provider supplying the service. We will also describe the more demanding case of post-payment. The following discussion is easiest to understand if we consider the accounts for one flow at a time. We assume that accounting is operating in nearreal-time on a per-session basis, thus such an approach makes sense as a session consists of a set of flows known to at least a subset of the participants. Again, for ease of understanding, let us consider one incoming flow to one customer before we consider her outgoing flow. Note that two or more edge-charges are raised per flow depending on the number of ends, but we are focusing on one customer at one end (termed ‘local’). For brevity, we will use feminine pronouns for the local customer and masculine for the remote customer(s). If the local customer doesn’t wish to pay her provider’s charges for reception, she will present the account of her received flow to the clearer (discovered using the procedure in the previous section). She will identify the remote sender who she expects will pay. The clearer looks up the sender’s ISP and debits its account, referring to the sender’s address. When the sender’s ISP settles its account with the clearer, it will deduct the amount from its account with the sender. The price used is the clearer’s, which may or may not be identical to the local provider’s price. If the sender is a large organisation, it might have an account directly with the clearer. The clearer also credits its account with the local ISP at the local provider’s price, referring to the local customer. When this account settles, it will clear the local customer’s debt with her local provider.

Whatever, the resulting intermediary could then be contacted to find the price being offered for the particular combination of ISPs. The same organisation would naturally take the payments and clear them between providers. The intermediary would also have to find out the prices being charged by the relevant edge-providers, which would represent its back-end costs. We assume the providers would be using tariff dissemination protocols such as in Rizzo et al [18], Carle et al [6] or Yemini et al [21] that could be listened to by the clearer as easily as by the edge-customers, particularly if transported over multicast. The sender cannot dispute the charge unless he has evidence that the receiver previously agreed to accept payment liability for traffic of this type from 9.2 Accounting and clearing with him. Purely for the clearer’s information, the local ‘sender liable but local payment customer may identify which records she believes the remote party expects to pay and which she is customary’ simply disputing. Thus despite the customary case We concluded earlier that only the sender should being each end paying its own charges, the onus is be ultimately liable for usage charges. However, we on the sender to ensure it can prove this. In most suggested that the customary position should be cases, there is a relationship between the parties to expect every customer to pay for both reception in a communication which will allow the sender to and sending. We will now describe how this neces- safely assume the customary case so that no resary but rather novel business model would work, ceiver is expected to bounce its customary charges assuming also end-to-end clearing. At this point we back to the sender. Where such trust isn’t present, have to make a clear separation between account- the sender might wish to ensure receivers have coning and liability for payment. We propose that firmed they are accepting the flow on the customeach edge customer and her edge network provider ary terms (where they pay their own end’s charges). should first reconcile their usage records for sent However, some ISP’s may offer sender debt collecc British Telecommunications plc, 2000

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tion as a service, bundling this liability with their 9.3 service.

We now move on to consider outgoing flows sent by the local customer. If she has evidence that the remote receiver has agreed to cover her local provider’s charges for her sent traffic, she will present her account for the sent flow to the clearer. The mechanics of looking up accounts and debiting and crediting are as before. The receiver can only dispute this charge if he can prove the evidence saying he agreed to pay is invalid.

The local customer may optionally notify her local provider that she expects settlement for each flow to come from the clearer not herself. If the clearer never pays the local provider, the local customer remains liable to her own provider for the charges. If she has also already paid the clearer, she can ask the clearer for evidence that it has settled for the usage in question with her local provider. If it can provide the evidence signed by the provider, her liability to her provider is cleared. If it can’t provide such evidence, it must return her payment so she will never have to pay twice.

This all seems very complicated, given it is per flow. However, in practice, settlement will be done in batch between each pair of parties as providers and clearers all have long term relationships with their customers. It is only the accounting that is perflow. Indeed, even the accounting may be done in batches, but each batch will contain per-flow granularity of usage records. Also, it is clear that such reapportionment will only be cost-effective for flows where the value of the communications quality is higher than the cost of reapportionment. Examples would be long-lived flows, or collections of flows between the same ends where premium quality transmission service is used. Also, it should be recalled, that the clearing intermediary is merely a role. This role may be taken by one of the edge-ISPs or by one of the edge-customers, which removes the need for half the messaging.

Inter-domain multicast with heterogeneous QoS

Illustrating the power of the principles set down so far, we can take an example like multicast with heterogeneous QoS per receiver and show that charging for it with correct apportionment will ‘just happen’, even inter-domain. However, it will only be efficient by using the ‘split-edge pricing’ model. For illustration, let us have two provider networks with an edge customer of Na sending into a multicast address where the tree crosses into Nb reaching receivers who are customers of Nb . Nb will have given a price for multicast reception and Na one for sending to an address in the multicast range. The tree may spread to other receivers on other networks too. The receivers in each domain will note when they join the multicast and each start being charged for the traffic they receive at their local price. The sender will be charged at her provider’s price. At the domain boundary between Na and Nb , Nb will be charged Na ’s price for sending to a multicast while Nb will charge its price to Na for receiving from a multicast. This usage may either be measured exactly at the inter-provider border, calculated statistically by combining customer usage data with multicast routing tables or simply covered within bulk measurements at the border. The receivers of a particular multicast group may happen to be located so that the multicast tree fans out immediately at its entrance to the network. With heterogeneous QoS per receiver (e.g. RSVP), any message to set up the QoS must emanate from the receiver and can therefore be charged for locally. Again this can be treated identically at the interprovider boundary. We believe edge pricing allows enough flexibility to charge differentially for broad ranges of route lengths because it allows different charges for different administrative domains. Even if a single domain spanned the globe, if desired it could be divided into internal pricing domains to achieve the same effect.

We now go on to discuss how reapportionment of charges between the ends might work in this multicast case. It would be unlikely that anyone would volunteer to pay for all receivers however many Essentially, receipts are being traded much in the there were, unless they had a prior arrangement same way as employees claim travel expenses from with each receiver. For instance, if a conference their employer. This results in a scenario where organiser were offering to pay everyone’s communithe traditional telephone bill becomes an anachro- cations expenses, part of the charge for each parnism. Such a bill represents two commercial pro- ticipant to join the conference would most likely cesses wrapped into one, which we propose should cover these costs. Thus, any number of particibe separate. The first stage is reconciliation of all pants could join but the host would still have all local usage records, whoever will pay. The second payments covered from income. The host might stage is agreement over who pays for which usage just allow enough in the conference charge to cover record. most prices of most providers and probably make a 10 of 20



10

service

PSTN

money

Internet

Limitations and Further Work

iterative

end-end 3

1 7

3

21

2 36 41

5

5 clearing

59

29 25

24

21

PIG 100

Figure 5: Clearing across a PIG little profit to cover the risk. Other models might be possible. The host may only agree to pay each participant’s charges up to a ceiling. Alternatively, the host may ask for receipts authenticated by ISPs and pay the exact charge of each ISP. Thus, the end-to-end pricing model allows full flexibility for reapportioning charges in both the multicast and unicast cases.

9.4

Phone to Internet gateway

Others are taking the approach of allowing the telephony charging model to determine that for Internet telephony. Because of the complexity implications this approach has, we suggest the Internet should take advantage of the opportunity for a fresh start. The Internet should reject the complexity of iterative PSTN charging before it becomes endemic. Instead, phone to Internet gateways (PIGs) should be treated exactly as endsystems are treated above. Any apportionment of sender and receiver payments should be dealt with from one end of the Internet to the PIG. That is end-to-end across the Internet’s patch, rather than end-to-end across both the Internet and the PSTN (Fig 5). This also allows for multicast models on the Internet side (multipoint on the PSTN side remains as complicated as today). Clearing between end-parties must never become muddled in with network provider pricing. If this were to happen, the Internet would be for ever saddled with interworking with a legacy, even when the legacy had virtually withered and died. It is always better to make the legacy interwork with the new model than the other way round.

The models described in this paper only become critical for any Internet communications that are usage-charged (e.g. QoS requests). Such cases are rare at present, however the author is engaged in related work, which suggests that the subscription and connect time charging models for Internet communications are only viable as long as capacity utilisation is low. However, even ultra-lightweight usage-charging [4] is yet to be proven cost-effective, therefore the context that this paper relies on is not at all certain without considerable further work. Also this paper argues the case for ‘sender and receiver both pay’, but it is not proven. The stability of inter-related ISP policies requires ‘war-game’ simulation as a step towards a proof. Further work is also required to exercise scenarios based on these models, through simulation and prototyping in order to fully work through the performance and security issues.

11

Conclusions

We have defined a generalised pricing model for any number of interconnected multi-service networks. Each network offers each of its neighbours each class of service at a separate price for each direction of transmission. We call this ‘split-edge pricing’. This model scales naturally to any size internetwork because all prices only depend on direct neighbours. We have shown that the common case for apportioning value between the ends of a connectionless communication network is catered for if all users are charged for both sending and receiving. We have also shown that this is the most stable and efficient case, particularly for multicast and aggregation. It should therefore be used as the default in the ‘split edge-pricing’ model.

We have suggested that a new business model would be useful and more efficient to cater for the cases where there is a large discrepancy from this default in terms of value apportionment — large enough for it to be worth the cost of making a balancing transaction. This new model requires a new role in communications markets — an intermediary for end-to-end pricing and clearing. This new role Note that the end customer will see no difference could be conducted by existing ISPs or customers if they rely on their edge network provider for all themselves, but there appears to be considerable Internet telephony charging. This is purely an in- added value, making this a viable business in its ternal re-arrangement between ISPs. However, the own right. It appears that this role is a threat customer could make these arrangements herself, if to existing ISPs business. The role is one suitshe desired. able for a purely financial processor using common c British Telecommunications plc, 2000

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e-commerce mechanisms with relatively low costs and the ability to take a share of the surplus value available on top of charge reapportionments. This role relegates edge ISPs into wholesalers for a potentially large class of Internet applications. The intermediary would become the retail face of the Internet in many cases. Further, we suggest a subtle twist to the recommendation that customers should pay for both sending and receiving. We suggest this should be customary, but that ultimate liability for sending should lie with the sender. Disputes could then quickly be resolved through the end-to-end clearing role. This stems from the unavoidable fact that receivers can’t avoid being sent to.

Acknowledgements Richard Gandon (BT International Carrier Services), Martin Tatham, Mike Rizzo, Jérˆ ome Tassel, Steve Rudkin, Chris Russell (BT Research), Jon Crowcroft (UCL).

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References [1] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, and W. Weiss. An architecture for differentiated services. Request for comments 2475, Internet Engineering Task Force, URL: rfc2475.txt, December 1998. [2] Bob Briscoe. The direction of value flow in connectionless networks. In Proc. 1st International COST264 Workshop on Networked Group Communication (NGC’99), volume 1736, URL: http://www.labs.bt.com/ projects/mware/, November 1999. Springer LNCS. (Invited paper). [3] Bob Briscoe. The direction of value flow in multi-service connectionless networks. In Proc. International Conference on Telecommunicatons and E-Commerce (ICTEC’99), URL: http://www.labs.bt.com/projects/ mware/, October 1999. [4] Bob Briscoe, Mike Rizzo, Jérˆ ome Tassel, and Konstantinos Damianakis. Lightweight, end to end, usagebased charging for packet networks. In Proc. IEEE Openarch 2000, pages 77–87, URL: http://www.labs. bt.com/projects/mware/, March 2000. [5] Frances Cairncross. The Death of Distance; How the Communications Revolution Will Change Our Lives. Harvard Business School Press, October 1997. [6] Georg Carle, Felix Hartanto, Michael Smirnow, and Tanja Zseby. Generic charging and accounting for valueadded IP services. In Proc. IEEE IFIP International Workshop on QoS (IWQoS’99) pricing workshop, June 1999. [7] David D. Clark. Combining sender and receiver payments in the Internet. In G. Rosston and D. Waterman, editors, Interconnection and the Internet. Lawrence Erlbaum Associates, Mahwah, NJ, URL: http://diffserv.lcs.mit.edu/, October 1996. [8] Ross Finlayson. Breadcrumbs. Minutes of Third Reliable multicast research group meeting, Orlando, FL, and following on the mailing list URL: http://www.east.isi.edu/rm/, February 1998. [9] Mark Handley and Van Jacobsen. SDP: Session description protocol. Request for comments 2327, Internet Engineering Task Force, URL: rfc2327.txt, March 1998. [10] Shai Herzog, Scott Shenker, and Deborah Estrin. Sharing the cost of multicast trees: An axiomatic analysis. Proc. ACM SIGCOMM’95, Computer Communication Review, 25(4), August 1995. [11] Hugh W. Holbrook and David R. Cheriton. IP multicast channels: EXPRESS support for large-scale singlesource applications. Proc. ACM SIGCOMM’99, Computer Communication Review, 29(4), September 1999. [12] Accounting rates and how they work. Technical report, ITU/Teleography Inc, URL: http://www.itu.ch/ intset/whatare/howwork.html, 1996. Extract of chapter 3, Box 3.1 from [13]. [13] The direction of traffic. Technical report, ITU/Teleography Inc, Geneva, URL: http://www.itu.ch/ti/ publications/traffic/direct.htm, 1996. [14] Reforming the international accounting rate system. Web site, ITU, URL: http://www.itu.ch/intset/, at latest 1998. [15] Nadia Kausar, Bob Briscoe, and Jon Crowcroft. A charging model for sessions on the Internet. In Proc. IEEE ISCC’99, Egypt, volume 1629, pages 0246–, URL: http://www.rennes.enst-bretagne.fr/~afifi/ iscc99.html, July 1999. Springer LNCS. [16] Jeffrey K. MacKie-Mason and Hal Varian. Some economics of the Internet. In Proc. Tenth Michigan Public Utility Conference at Western Michigan University, URL: http://www.sims.berkeley.edu/~hal/people/ hal/papers.html, March 1993. [17] Sean McCreary and kc claffy. How far does the average packet travel on the Internet? Technical report, CAIDA, URL: http://www.caida.org/Learn/ASPL/, May 1998. [18] Mike Rizzo, Bob Briscoe, Jérˆ ome Tassel, and Kostas Damianakis. A dynamic pricing framework to support a scalable, usage-based charging model for packet-switched networks. In Proc. Int’l W’kshp on Active Networks (IWAN’99), volume 1653, URL: http://www.labs.bt.com/projects/mware/, February 1999. Springer LNCS. [19] Scott Shenker, David Clark, Deborah Estrin, and Shai Herzog. Pricing in computer networks: Reshaping the research agenda. ACM SIGCOMM Computer Communication Review, 26(2), April 1996. [20] Tony Speakman, Dino Farinacci, Steven Lin, Alex Tweedly, Nidhi Bhaskar, Richard Edmonstone, Kelly Morse Johnson, Rajitha Sumanasekera, Lorenzo Vicisano, Jon Crowcroft, Jim Gemmell, Dan Leshchiner, Michael Luby, Todd Montgomery, and Luigi Rizzo. PGM reliable transport protocol specification. Internet draft, Internet Engineering Task Force, URL: draft-speakman-pgm-spec-05.txt, November 2000. (Work in progress) (expires 24 May 2001). [21] Y. Yemini, A. Dailianas, and D. Florissi. MarketNet: A market-based architecture for survivable large-scale information systems. In Proc. 4th ISSAT International Conference on Reliability and Quality in Design, Seattle, WA, URL: http://www.cs.columbia.edu/dcc/marketnet/, August 1998. c British Telecommunications plc, 2000

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[22] Lixia Zhang, Stephen Deering, Deborah Estrin, Scott Shenker, and Daniel Zappala. RSVP: A new resource ReSerVation protocol. IEEE Network, September 1993. [23] H. Zimmerman. OSI reference model — the ISO model of architecture for open systems interconnection. IEEE/ACM Transactions on Networking, 28(4):425–432, April 1980. [24] Chris Zull. Interconnection issues in the multimedia environment. In Proc. Interconnection Asia ’97, IIR Conferences, Singapore, URL: http://www.cutlerco.com.au/core/content/speeches/Interconnection\ %20Iss\%ues/Interconnection_Issues.html, April 1997.

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Nd

Ne

B

There follows analysis that will produce a general model for the surplus (or deficit) income that any party on a network can expect for any topology (unicast or multicast). Also pricing can be different for traffic in each direction (note that reverse direction multicasting is aggregation, which is catered for in the model).

Nb W Wabr bas Na

Wbar Wabs

Nc Nc

Figure 6: Split-edge pricing per class of service

A

Independence of classes of service

logical

Here we argue that each class of service can be treated independently of other logical classes of service. Fig 6 attempts to show the split-edge prices for different classes of service between Na and Nb by layering the diagram in the third dimension ‘out of the page’. Each class of service ‘layer’ is a logical independent inter-network in its own right as it has its own share of resources and its own internetwork prices. Relating this to currently proposed technology, for integrated services [22], class of service is defined as either best effort, controlled load, or guaranteed service with the particular flowspecs reserved being dealt with as heterogeneous QoS within a class (Appendix B). For differentiated service (DS) [1], each DS code-point represents a class of service.

However, because each network is managed autonomously, there may be disjoint mappings between classes of service in neighbouring networks (as allowed in diffserv). Such a case is shown between Nb and the right-most Nd , which uses one class of service where everyone else uses two. At such a boundary, Wdbs and Wdbr for the merged class appear from Nb ’s point of view to each be a pair of prices for each of its classes of service that happen to be identical. On the other hand, Nb might offer two different Wbdr prices and two Wbds prices to Nd for each of Nb ’s two classes. Nd would just see each pair as a single price that varied depending on the relative proportions of traffic coming from or going to each class. Therefore, even with disjoint class mappings, we need not concern ourselves with more than one class of service at a time. c British Telecommunications plc, 2000

Split-edge pricing model

Nd

We make the assumption that differences in length of routes or shapes of routing trees through any party’s network will not produce cost differentials that are significant enough to be worth measuring and charging for. I.e, we assume the ‘death of distance’ [5], or a ‘black box’ network routing assumption where a network’s routing is hidden within its interfaces. This means we trust a network or internetwork of federated domains to find the cheapest route without end-system intervention or, put another way, that routing protocols approximate to a competitive market. If the route isn’t truly the cheapest, we assume the discrepancy is so minor that the end-customer isn’t concerned. Border routing policy distorts this considerably, but one of our long term motivations is to make most border-routing policy redundant, simplifying interprovider interfaces using usage-charging instead. This also implies that internal network design inefficiency should be absorbed by providers in their overall pricing. For instance in current multicast routing, tree stability always takes priority over tree efficiency. Providers are free not to make this decision if they can design better multicast routing algorithms, but there is no need to expose internal cost differentials in external pricing (when they are purely for provider convenience in the first place). This is deliberately unlike any of the scenarios analysed in Herzog et al — the classic work on sharing the cost of multicast [10]. Herzog et al has a scenario where all receivers share the cost equally, but not where receivers are divided into sets based on provider domains and only charged equally per domain (edge pricing). Nor does it consider heterogeneous QoS. If some degree of distance-based pricing is required, we assume edge-address-based pricing mechanisms can be used without having to concern the end-systems with the route between addresses. But we believe even this is unlikely. Fig 7 shows a generic topology that will be used to illuminate the analysis. The general model and terminology have already been introduced in Section 5. We explained the scenario of a packet or flow being multicasted between inter-connected networks of various statuses relative to the one of interest and we explained the four price weights at any interface. However, we glossed over the details of the 15 of 20


model (e.g. not saying what the weights were applied to). We will now correct those omissions. The figure shows the classes of service, Q, set for each branch of the tree. Q may be confined to discrete levels or allowed to take any from a bounded continuous range of values, depending on the QoS mechanism. It is assumed that, wherever a packet is duplicated for multicasting, the multiple copies might each have different classes of service. Note that it has not been assumed that the branches all have different classes of service. This depends on the (independent) requirements of the ultimate ends of each branch. There need be no correlation between neighbouring network status and the value of Q for that branch. RSVP is an example of such a heterogeneous QoS scheme (diffserv has the potential to become heterogeneous, e.g. by setting the class of each branch based on the DS-bytes in the multicast routing packets or in the IGMP join packets from end systems or even using RSVP). The packet or flow being modelled could be data or signalling. V represents some measure of the volume or size of the service consumed. It might be the amount of data in the packet or in a flow of similar packets for a certain time. It might be the time for which a reservation of a certain size is held. We simplify the model by requiring V to be the same for all branches of the tree. This is justified because a branch leaving the tree, a packet loss or a network filter can be dealt with as an alteration to the topology rather than allowing V to be heterogeneous. We define the ’nominal charge’ function that Nb levies for the packet or flow as Cb (V, Q). This is a nominal charge because next we will describe how it is weighted to determine the actual charge for each different type of neighbour. Wbas & Wbar denote the per direction weightings applied to the charge that Nb applies to Na as shown at the highlighted interface between these two in Fig 7. The third digit of the suffix denotes the direction of traffic that the weighting applies to; s being the weight for traffic sent into the provider, r for traffic received from the provider setting the charge. However Na is also offering service to Nb . So similarly Na weights its charge Ca with weights Wabr and Wabs . The first digit of the suffix of W denotes the network provider setting the charge being weighted. The second digit denotes the type of neighbour network provider to which this weighting applies. Often, we can consider scenarios where many of these price weightings are set to be either equal to each other or to be zero, but formulae derived for this model allow fine granularity of price weighting for any scenario we might dream up in future 16 of 20

Nd

Ne

Qv

Nd Qw

Qu

Qw

Nb Wbas Wabr Na V

Qt

Qu Wbar Wabs

Nc Qu

Nc

Figure 7: Split-edge pricing with heterogeneous QoS work. We have initially experimented with extreme policies like ‘sender pays all’ but the formulae allow consideration of more subtle scenarios where prices might be slightly unequal in the different directions, perhaps because of asymmetric access technology like xDSL. To save drawing multiple diagrams, we will associate two integer direction flags with each packet. The forward direction flag, d, will be 1 if the direction is as shown in the figure and 0 if reversed. We define ð = (int)(¬((boolean)d)). That is, ð is effectively the negation of d (that is ¬d), but they are both integers, hence the need to cast d to boolean before negating it then casting it back to an integer. ð can be thought of as the reverse direction flag and is always the opposite of d. These two flags toggle on or off different terms in all the following formulae. Thus, the networking surplus that Nb makes from Na (also taking into account what Na charges Nb ) is: Sba = d[Cb (Wbas , V, Qt ) − Ca (Wabr , V, Qt )] + ð[Cb (Wbar , V, Qt ) − Ca (Wabs , V, Qt )]. (1) Note that the networking surplus is only the surplus from running the networking layer business. This surplus also has to cover the cost of lower layer infrastructure and the cost of charging. The cost of these aren’t modelled here as they are purely local to the provider and they are far less dynamic than the per-packet or per-flow approach being taken. Therefore they can be dealt with on far longer timescales. Although this is the general formula, for the scenarios we shall be analysing here there is too much generality to get a useful result. In practice, the following formula would be a reasonable specific form c British Telecommunications plc, 2000


of the above: Sba = V [(dwbas + ðwbar )pb (Qt ) − (dwabr + ðwabs )pa (Qt )].

set certain weightings to zero to represent scenarios where one provider doesn’t charge at all where another does. (2)

This simplification is based on two assumptions, which move all the non-linearity into a new ‘nominal pricing’ function, p(Q):

For brevity, we define ωijs = dwijs + ðwijr for all i,j. Therefore formula (2) can be written: Sba = V [ωbas pb (Qt ) − ωabr pa (Qt )].

(2∗ )

∗ 1. Both charge functions vary linearly with the Using formula (2 ) based on these assumptions, the volume of the service consumed. That is, for networking surplus that Nb would make from the multicast shown in Fig 7, taking into account the all i, j, k, z: charges it makes to neighbouring networks and they make to it, would be: Ci (Wijz , V, Qk ) = V Pi (Wijz , Qk ),  ! where Pi is the price per unit V (which leads X X to charge, Ci ). Sb = V ωbas pb (Qt ) + ωbjr nbjk pb (Qk ) j∈C

2. Each provider has a single nominal charging X function for all its customers and only varies − ωabr pa (Qt ) − the price for each status of customer around j∈C this. We can then define a new set of weightings, w (lower case), which subsume the old weightings, W (upper case), to ensure all the pricing functions of one provider are the same. where, using Fig 7: That is, for all i, j, k, z: Pi (Wijz , Qk ) = wijz pi (Qk ), where pi (Qk ) (lower case) is a pricing function for service class Qk to which all offers by Ni are linearly related. We would have to undo the first simplification if we wanted to analyse volume discounting. We would have to undo the second simplification if we wanted to analyse network providers who wanted to present different price-quality sensitivity to different neighbours. We assert that these are not unrealistic steps. We have included the pricing function with respect to quality, p(Q), and direction sensitive weightings, which capture the two essential differences between the future multi-service Internet and the fixed quality ’phone network. Our simplifying assumption that the price-quality function has the same non-linearity for all neighbours appears justified in comparison with the international accounting rate system (IARS) [14] for the telephone network. Completely linear weightings (called international accounting rates) are used between each pair of international carriers for a service which sits ostensibly at a single point on the quality axis. Further, even our simplified model contains yet a third subtle but important improvement over IARS. It allows two independent weightings for each flow direction between each pair of networks as opposed to one. This allows each provider to set its price for the other independently without each having to negotiate a price with the other. And we can still c British Telecommunications plc, 2000

k∈U

ωjbs

X

! nbjk pj (Qk ) 

k∈U

(3)

• the set, C, of suffixes of networks connected to Nb with a branch (not root) of the multicast would be C = {c, d, e, . . .}; a ∈ / C; • the set, U, of suffixes of classes of service of branches would be U = {u, v, w, . . .}; t ∈ / U; • and nbcu is the number of branches of the multicast routing tree that flow from Nb to Nc with class of service Qu . Note, we are not implying there is any need for Nb to calculate its networking surplus on a per-packet or per-flow basis. In fact, quite the opposite; the model is designed to avoid the necessity of any centralised control for each domain other than pricing on a longer term basis. We merely need our formula for a network domain’s surplus in order to analyse policy stability. Also, in practice, the class of service at the head of the tree need only be the maximum of the set of classes of service of any branch, {Qk | k ∈ U } (due to the logic of either multicast or aggregation): Qt = max(Qk ).

(4)

To illustrate the flexibility of formula (3), let us now consider an intra-provider unicast (between two edge-customers, say both Nc , of the same provider, Nb — often termed ‘on-net’). 17 of 20

The Direction of Value Flow Unicast implies that formula (4) reduces to: Qt = It can be seen that for intra-provider packets, the max(Qu ) = Qu and that: revenue is unaffected by whether senders, receivers or both are charged, but which customer(s) connbjk = 1; when j = c and k = u; tributes to it, obviously depends on the direction of the unicast. nbjk = 0; otherwise. Nc being edge customers implies: wcjz = 0;

for all j, z.

Therefore, (3) collapses to: Sb = V (ωbcs + ωbcr )pb (Qt ), = V (d + ð)(wbcs + wbcr )pb (Qt ), = V (wbcs + wbcr )pb (Qt ) (because (d + ð) = 1); Sc = −V ωbcs pb (Qt ).

However, for inter -provider packets where peer providers charge each other as much as any edge customer, if both providers adopt a ‘receiver pays all’ or a ‘sender pays all’ policy, the revenue ends up always moving to the edge provider furthest from the customer paying. As long as each provider has a similar customer mix in terms of senders and receivers the net effect is that all providers make similar gains (and incidentally they could mutually agree not to charge each other with little change (5) in their income — peering). Charging for only one direction has the benefit of halving the cost of mea(6) suring and accounting for both directions.

Next, let us consider an inter -provider unicast (between two edge customers both Nc , but of different providers, Na and Nb — often termed ‘off-net’) consuming quality, Qt , with Na upstream of Nb when d = 1. The networking surplus of each party is: Sa Sb Sca Scb

= V [(ωacs + ωabr )pa (Qt ) − ωbas pb (Qt )]; = V [(ωbcr + ωbas )pb (Qt ) − ωabr pa (Qt )]; = −V ωacs pa (Qt ); = −V ωbcr pb (Qt ).

(7) (8) (9) (10)

We make one further simplifying assumption, in order to focus on just first order effects. That is, we assume Na and Nb set similar nominal price functions pa (Q) & pb (Q) for all practical Q. We shall denote either function as just p(Q). This is unlikely to be a valid assumption as ISPs are likely to build their brand identity around their price-quality sensitivity, however this will cause second order effects that would otherwise obscure the purpose of the current work.

However, if all provider policies start as ‘sender pays all’, Na might notice it has more receivers than senders. Let us assume Na switches to charging receivers only, while Nb continues to only charge senders. A packet in the direction from Nb to Na would result in each provider charging their edge customer for Qt , but neither charging each other. A packet in the opposite direction would result in neither provider charging their end customers at all, but both nominally charging each other. This would, as one might expect, cause a migration of all predominant receivers to other providers like Nb and all big senders to Na . This leaves all providers with very little edge revenue, but all just nominally charging each other similar amounts. Clearly this will not be tolerated for long as the edge-customers are exploiting the providers, all the traffic is being inefficiently and expensively funnelled across the inter-provider links and there is consequent pressure for all providers to reverse their policy. Changing the wholesale pricing policies will make no difference. If the retail policies remain imbalanced there will be little revenue entering the system. Thus neither all providers charging only for sending nor all charging only for receiving can be stable unless there is some industry-wide agreement as to which one to standardise around. If there is such tacit agreement, charging for sending seems preferable as it avoids the problem of charging for unsolicited receipt.

With these simplified formulae for unicast, we can first show how the formulae can be used to explore some possible unicast charging scenarios then consider their commercial implications. To help to connect together the scenarios, we will use numeric values for the weightings that could be used in realistic scenarios. To help further, weightings for all scenarios will be normalised relative to an edge customer, What this teaches us is that any extreme policy Nc , such that wicz = 0, 1 or 2 for all i & z wherever where either sending or receiving are offered at low or zero price encourages instability simply because possible. local pricing doesn’t match true local costs. ThereTable 1 gives the networking surplus (deficit) for fore, when end customers arrange themselves to each party in various scenarios. In the first scenario, their best advantage, providers suffer unless they as there are two edge customers, Nc , of the same collectively organise themselves, which is unlikely. provider, Nb , the edge customer surplus, Sc1 , is the On the other hand, all providers charging for both sender’s and Sc2 is the receiver’s when d = 1. sending and receiving is also stable, and without 18 of 20



Scenario (English)

Scenario (maths)

single provider, Nb , with edge customers, Nc , unicasting.

Sb V pb (Qt )

Sc1 V pb (Qt )

Sc2 V pb (Qt )

(5)

(6)

(6)

. . . and receiver pays all

wijs = 0; wijr = 2;

any i, j

2

−2ð

−2d

. . . or sender pays all

wijs = 2; wijr = 0;

any i, j

2

−2d

−2ð

. . . or senders and receivers pay equally

wijs = 1; wijr = 1;

any i, j

2

−d − ð = −1

−d − ð = −1

Sa V p(Qt )

Sb V p(Qt )

Sca V pa (Qt )

Scb V pb (Qt )

(7)

(8)

(9)

(10)

two providers, Na & Nb each with an edge customer Nc , unicasting. retail & same. . .

wholesale

prices

. . . and all retail prices identical. . .

wacz = wabz ; wbcz = wbaz ; for either z wacz = wbcz ; for either z


wijs = 0; wijr = 2;

any i, j

2d

2ð

−2ð

−2d


wijs = 2; wijr = 0;

any i, j

2ð

2d

−2d

−2ð


wijs = 1; wijr = 1;

any i, j

d+ð =1

d+ð =1

−d − ð = −1

−d − ð = −1

2d

2d

−2d

−2d

. . . or Na only charges senders and Nb only charges receivers. . . . . . but all at the same price wholesale prices are half retail. . . . . . and all retail prices identical. . .

wajs = wbjr = 2; wajr = wbjs = 0; 2wacz = wabz ; 2wbcz = wbaz ;

any j

for either z

wacz = wbcz ; for either z


wics = 0; wicr = 2;

any i

d+ð =1

d+ð =1

−2ð

−2d


wics = 2; wicr = 0;

any i

d+ð =1

d+ð =1

−2d

−2ð


wics = 1; wicr = 1;

any i

d+ð =1

d+ð =1

−d − ð = −1

−d − ð = −1

2d

2d

−2d

−2d

. . . or Na only charges senders and Nb only charges receivers. . . . . . but all at the same price

wacs = wbcr = 2; wacr = wbcs = 0

Table 1: Unicast price weighting scenarios


19 of 20


artificial standardisation. If any provider breaks ranks and charges, say, only senders, receivers will quickly migrate towards it and senders away to the rest of the industry. Thus the most stable arrangement is for send and receive pricing to approximately match costs.

one tree. All receivers are just usage-charged as they occur, with no need for co-ordination or totalling. As before, if the sender (or one receiver, or even a third party) wants to offer to pay for all receivers, this can be arranged end-to-end, rather than through the networks.

When multicast is analysed, this only strengthens the case for all ends being liable for their use of the network, whether sending or receiving. Here we consider single-source trees for brevity, but the argument is even stronger for shared trees. Today, multicast senders are being charged exorbitant rates such that only those with income from (or advertising to) large numbers of receivers become customers. In the longer term, ‘sender pays all’ will only be tenable if it is at a rate that reflects the number of receivers. However, if this problem is considered, the trust required to achieve a solution appears to put up a theoretical barrier to ‘sender pays all’. The cost of an inter-domain multicast is spread throughout the tree. The cost to each domain is approximately proportional to the number of branches within that domain. The proximity of the tree’s fan-out to its ingress into the domain determines the actual cost per branch. This in turn depends on how meshed the network is, which is usually a characteristic of the design of a whole domain (and if not, the internal design is under the domain’s control). Thus, a ‘leaf-domain’ (one with only edge receivers and no downstream domains on the tree) can work out its cost for a certain number of receivers in its domain and report it to the upstream domain. This report is effectively a bill presented to the upstream domain, requesting a share of the income from the sender as it trickles down through the domains in the same direction as the tree. This upstream domain can collect similar ‘bills’ from other downstream networks and report the sum onwards upstream, eventually reaching the sender. Thus the head-end provider charges the sender the costs of the whole tree and has to pass on much of this income to meet the downstream ‘bills’. Clearly, any domain can overreport its bill and make a profit. This is because the sender cannot determine the topology directly from its receivers without destroying the scalability benefits of multicast, which deliberately hides receiver activity. It seems far simpler to apply the same argument as for unicast above and charge each receiver and the sender. The charges need not be identical for each, but the sender charge doesn’t need to reflect the number of receivers. Where the tree crosses a domain boundary, each domain simply charges the other as one sender or one receiver. This ensures charging is completely distributed. There is not even a need to correlate together the receivers for 20 of 20
